1. Field of the Invention
The present invention relates to an infrared-absorptive compound and an optical recording medium comprising a recording layer containing this infrared-absorptive compound.
2. Related Background Art
In general, optical disks and optical cards comprise a substrate and a thin recording layer provided thereon and can record high-density information in the recording layer in which optically detectable small pits (with size of, for example, about 1 .mu.m) are formed along a spiral, circular or linear track(s). To write information in such optical disks or optical cards, the surface of a laser light-sensitive layer is scanned with a converged laser beam so that pits are formed at the points irradiated with this laser beam, along a spiral, circular or linear track(s).
The laser light-sensitive layer is capable of absorbing laser light energy to form optically detectable pits. For example, in a heat mode recording system, the laser light-sensitive layer absorbs heat energy, so that small cavities (pits) can be formed by evaporation or melt at the energy-absorbed area. In another heat mode recording system, absorption of energy of emitted laser light causes the changes in optical density at the energy-absorbed area to form a pit.
A thin film of an organic coloring matter may be used as a recording layer of an optical recording medium so that the optical contrast of recording pits can be set higher. For example, a polymethine type, azulene type, cyanine type or pyrylium type coloring matter having a high absorptivity for the laser light may be used for the thin film, whereby a light-absorptive reflecting film showing a metallic gloss (reflectance: 10 to 50%) can be obtained to give an optical recording medium that enables the recording with laser beam and reading out by reflection. In particular, when a semiconductor laser with an oscillation wavelength of from 600 to 900 nm is used as a laser light source, the apparatus can be made smaller in size and cost. The thin film of organic coloring matter, however, is liable to change with heat and light causing the lowering of recording-reproducing performance and storage stability problems.
In order to prevent the lowering of recording-reproducing performance and storage stability, an optical recording medium has been proposed, as in U.S. Pat. No. 4,656,121, comprising an organic coloring matter thin film in which an infrared-absorptive compound such as an aminium salt or diimonium salt compound is added. Such a conventional aminium salt or diimonium salt compound can prevent the lowering of recording-reproducing performance and storage stability, but there is a problem that the amount of the aminium salt or diimonium salt compound in the thin film of the organic matter is so large that reflectance is lowered resulting in the lowering of the recording sensitivity of the optical recording medium.
The present applicants have already made some proposals in U.S. Pat. Nos. 4,923,390, 5,009,987 and 5,024,917, to solve the above problems in the prior art.